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LTC1871-1 Datasheet(PDF) 17 Page - Linear Technology |
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LTC1871-1 Datasheet(HTML) 17 Page - Linear Technology |
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17 / 36 page  LTC1871-1 17 18711fb APPLICATIONS INFORMATION For many designs it is possible to choose a single capacitor type that satisfies both the ESR and bulk C requirements for the design. In certain demanding applications, however, the ripple voltage can be improved significantly by con- necting two or more types of capacitors in parallel. For example, using a low ESR ceramic capacitor can minimize the ESR step, while an electrolytic capacitor can be used to supply the required bulk C. Once the output capacitor ESR and bulk capacitance have been determined, the overall ripple voltage waveform should be verified on a dedicated PC board (see Board Layout section for more information on component place- ment). Lab breadboards generally suffer from excessive series inductance (due to inter-component wiring), and these parasitics can make the switching waveforms look significantly worse than they would be on a properly designed PC board. The output capacitor in a boost regulator experiences high RMS ripple currents, as shown in Figure 12. The RMS output capacitor ripple current is: IRMS(COUT) IO(MAX) • VO –VIN(MIN) VIN(MIN) Note that the ripple current ratings from capacitor manu- facturers are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor or to choose a capacitor rated at a higher temperature than required. Several capacitors may also be placed in parallel to meet size or height requirements in the design. Manufacturers such as Nichicon, United Chemicon and Sanyo should be considered for high performance through- hole capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest product of ESR and size of any aluminum electrolytic, at a somewhat higher price. In surface mount applications, multiple capacitors may have to be placed in parallel in order to meet the ESR or RMS current handling requirements of the application. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount packages. In the case of tantalum, it is critical that the capacitors have been surge tested for use in switching power supplies. An excellent choice is AVX TPS series of surface mount tantalum. Also, ceramic capacitors are now available with extremely low ESR, ESL and high ripple current ratings. Boost Converter: Input Capacitor Selection The input capacitor of a boost converter is less critical than the output capacitor, due to the fact that the inductor is in series with the input and the input current waveform is continuous (see Figure 12b). The input voltage source impedance determines the size of the input capacitor, VIN LD SW 12a. Circuit Diagram 12b. Inductor and Input Currents COUT VOUT RL IIN IL 12c. Switch Current ISW tON 12d. Diode and Output Currents 12e. Output Voltage Ripple Waveform IO ID VOUT (AC) tOFF ΔVESR RINGING DUE TO TOTAL INDUCTANCE (BOARD + CAP) ΔVCOUT Figure 12. Switching Waveforms for a Boost Converter |
Similar Part No. - LTC1871-1_15 |
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Similar Description - LTC1871-1_15 |
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